Screen printing photostencil systems, as presently known, are almost entirely based upon the so-called "indirect" or "direct" photostencils, or upon a "direct/indirect" variant which combines some of the aspects of each. The indirect or transfer photostencil utilizes a light sensitive emulsion which is precoated on a paper or plastic base, and sensitized by bichromate or iron salts. When held in place relative to a film master by a vacuum frame or similar device, the sensitized film is exposed to actinic light. Thereafter the unexposed emulsion is washed away. Following this, the stencil can be mounted onto a screen, and after drying the paper or plastic base can be stripped away.
Indirect photostencils are thus applied to one surface of a screen, and although they offer high resolution (high print quality) because the thin stencil image can lie flush against a printing surface, they also have limited durability (due to the brittleness and low abrasive resistance) and are seldom used for more than a few hundred prints. The process for preparing such stencils is time consuming and critical in many respects. For example, the fabric preparation, sensitizing if required, washing and drying sequences can take up to an hour, and have to be conducted under controlled conditions and without exposure to ambient light. Further, the exposure time has little latitude and the temperature and humidity may also have to be controlled to provide a useful stencil. The need for a vacuum frame or comparable holder cannot be avoided.
In the direct photostencil, after sensitizing, the photosensitive emulsion is coated on both sides of the mesh, although some expertise is needed to insure an even coat. In order to obtain an adequate thickness this is usually done by successive drying and coating steps, each of which take a substantial amount of time. The direct photostencil is substantially more durable than the indirect stencil, and can be used to make thousands of copies, but with substantially lower resolution than the indirect photostencil. Because the surface of the emulsion tends to follow the contour of the mesh upon shrinkage when water is dried off, there is poor contact with the printing stock and a decrease in print quality, and if the photostencil is not carefully prepared, quality is lost for this reason as well. In any event, the direct photostencil also requires substantial drying time or the use of special drying procedures, and a substantially longer exposure time than does the indirect photostencil.
The so-called direct/indirect photostencil provides a flat surface on one side of the mesh by applying a precoated base that is laminated to the mesh with a sensitized emulsion coated on the inside of the screen. This laminate is then dried and the support layer is stripped away, and the mesh is then coated as in the direct system. This technique provides a resolution approaching that of the indirect system, because the flat base surface on one side can be flush with the printing stock and because there can be more consistent control of thickness. However, the time taken to prepare the sensitized coating, the limited shelf life, drying time, and the need for a vacuum frame still constitute drawbacks, and in addition the direct/indirect method is substantially more expensive than the other techniques.
The state of the art in preparing high quality photostencils looks to a number of practical factors that must be evaluated in considering any competitive system. It is desirable to avoid specialized coating steps that require a high level of skill and experience, as well as the use of chemicals having inherent instabilities and limited shelf life. Capital costs are of importance, and in this respect it is desirable to avoid the usage of vacuum frames and special light sources. It would also be desirable to obtain the resolution of the indirect system, with the durability of the direct photostencil, or to improve on each of such properties, but without imposing a cost penalty. In addition, some sensitizers such as the widely used dichromates present occupational health problems because of their toxicity. Other sensitizers present occupational health problems because they incorporate substantial amounts of solvents which must be evaporated and therefore present significant contamination problems in a closed processing room.
The present state of the art seeks to have a stencil exhibiting minimal "sawtoothing", or a tendency to follow the outlines of successive intersections of the mesh rather than the precise curvature or line defined by an edge of the film master. The sawtooth effect is dependent upon the mesh size, the material used and the orientation of the image edge to the filaments of the mesh. The stencil must have a good "bridging" property, and follow the curved or straight edge of an image in traversing across the openings of the screen. The photostencil should also be free of "undercutting" to the greatest extent possible, this effect arising when light scattering causes exposure within marginal image areas that are not intended to be exposed. The quality of the resolution of a photostencil can be determined by using a master image having test lines and patterns involving different resolutions. A photostencil is considered as having high resolution when it can be used to reproduce lines of 0.1 mm width with less than 0.01 mm undercut. A photostencil is also considered to be adequately durable if it can be used for making a few thousand copies, and withstand several washouts. It is evident, however, that it would be preferred if a substantially greater number of copies could be made. In addition, it is often desired to be able to remove the stencil image, and to reuse or "reclaim" the screen once or repeatedly for successive stencil images, or store screens for future use without deterioration. Common difficulties in this respect are excessive brittleness and water absorption.
The particular requirements that are encountered in making photostencils thus may be viewed in terms of preparing a precision, three dimensional but planar composite of filamentary screen and adhering stencil. This composite must have precise relationships between the screen and the stencil in the thickness dimension, along the edges of the stencil, and throughout the two dimensional pattern constituting the stencil image. The resultant image must adhere to the screen firmly, throughout many printings, even though the image may include extremely small incremental areas. For example, a stencil dot may approximate in area the area of the individual opening of a very fine mesh as long as it is secured by two filaments in each direction. Moreover, the preparation of a photostencil is essentially a manual operation, and one that must be adaptable to a wide range of conditions. For example, it may be necessary to prepare stencils on a wide variety of screen materials, and in sizes ranging from 16 to 508 strands per inch and more. The time and skill required to prepare the screen for exposure are both important in terms of labor costs. The equipment and techniques required to retain and expose the stencil are only partially determinative of capital costs. One must also consider the need for venting or control of toxic fumes, the usage of drying equipment and techniques for limiting light exposure. A major practical problem is that photostencil preparation can involve the usage at different times of many different mesh sizes, fabric types, and printing inks to meet particular image sizes and resolutions and specific printing requirements.
Workers in the art have not heretofore used other, radically different photosensitive materials, in preparing photostencils on a commercial basis. It has been suggested, as in U.S. Pat. No. 3,891,441, to employ photopolymerizable layers on opposite sides of a backing support, typically a printing paper. Although it is suggested that this might also comprise a mesh screen of nylon or silk, the only examples given are of thin paper for mimeographic use. The principal concept is to use the cover layers partially to define the thickness of the polymerizable layer (e.g. 40 microns including the paper) and to permit the cover layers to be stripped freely from the exposed regions, while adhering to the unexposed and unpolymerized regions, so that only the desired image is retained on the printing paper. Using stencils thus prepared in a mimeographic or hand rotary printing process, 500 and 700 copies were all that were reproduced. U.S. Pat. No. 3,891,441 also describes the incorporation of a substantial amount of filler material, such as powdered zinc oxide or carbon black, for the purpose of controlling the adhesion of the photopolymerizable layer to the cover layer. The techniques of this patent obviously do not satisfy the requirements of the modern photostencil industry, in terms of precision or durability, and although the exposure times required for thin photopolymerizable layers are relatively short, the need for a vacuum frame still exists.
The use of a photopolymerizable substance to prepare a relief image is discussed in U.S. Pat. No. 3,297,440, in terms of planographic printing plates as for offset printing methods, matrices for printing matter, silk screen printing stencils, and plastic or metal printing cylinders. However, no examples are given of the manner in which silk screen stencils can be prepared and the only examples of the preparation of an image from a film master involve the use of an aqueous solution as a layer upon a backing film, following which a process transparency is laid over it, and the three elements in the laminate are then pressed between two glass plates for illumination of the light sensitive layer through the transparency. This then provides a relief image, but there is no suggestion as to how a screen stencil meeting the requirements of the modern art could be provided.
U.S. Pat. No. 3,658,529 proposes the fabrication of photographs, using photocurable compositions that are sensitive to ultraviolet light, instead of conventional silver photographic emulsions. It is suggested that the photopolymer can be of any viscosity within a wide range, and that it be disposed on a porous essentially transparent layer such as a film or a fiber sheet. The objective is simply to provide a base for supporting the photographic image, and there is no showing or suggestion of how an image could be prepared that would be suitable for providing a screen-based composite image of the type required for the photostencil industry. Thus examples 2, 3 and 22-27 of the patent deal with the preparation of positive half tone images on various substrates. There is no confrontation of the problem of retention of the film master or preparation of a durable but extremely precise stencil image.
Another mention of the usage of a photopolymer to prepare a stencil image is contained in U.S. Pat. No. 3,826,650. In this patent, which deals primarily with the photosensitizer employed, it is suggested that a paper or silk screen stencil can be provided by impregnating a layer with the photopolymer solution, exposing through a negative, and removing the unexposed liquid or monomer. A similar general discussion is contained in U.S. Pat. No. 3,961,961 which additionally proposes that the photosensitive layer can be given a tacky characteristic, such that it will adhere to adjacent support and release layers that are used prior to attachment of the photosensitive layer to a screen or paper. Although it is mentioned that the screen may be reclaimable through the use of an acidic acid solvent, it is also specifically required to utilized a vacuum frame for holding the film master. It is noted for completeness that subsequent to the filling of the parent of this application, Belgian Pat. No. 869,190 issued on Nov. 16, 1978, proposing largely the use of certain chemical mixtures for preparing stencils from photopolymer systems.
It is also generally known to make printing plates from photopolymers, as evidenced by U.S. Pat. Nos. 4,137,081, 4,139,436, 4,022,674, 4,011,084 and 3,982,492. There are, however, fundamental differences between the preparation of printing plates and photostencil screens. The printing plate has a raised image protruding from the cylinder base or other substrate. It is desirable, therefore, in these systems to provide a high hardness but low brittleness, and to have tapered sides so as to provide better structural support for the dots or elements forming the printing image to meet other specific requirements concerned with that industry.
In the preparation of printing plates, as evidenced by U.S. Pat. Nos. 2,791,504, 3,615,450, 3,837,887, 4,048,035 and 4,147,549 it is common practice to employ an anti-halation layer between the base and the photopolymer layer, which is usually retained within a frame or well. The thickness of the layer is determined by the dimensions of the peripheral gasket, frame or shims that are used, with or without a doctor blade or an intermediate layer such as a plastic film or a glass sheet. Although the master film itself may be coated with a protective layer and placed directly in contact with the photopolymer layer, it usually is preferred to incorporate an air gap and to expose using a source which enhances the tapered relief characteristic that is desired. Generally, a wide band light source in the wavelength region of 200 to 400 nanometers is employed. The art includes a number of suggestions as to dyes and pigments, including carbon black, that might be used within the anti-halation layer underneath the relief image, or in the photopolymer itself, for general purposes that are often contradictory in nature. Thus U.S. Pat. No. 2,791,504 suggests that dyes and pigments in the photopolymer serve no useful purpose because they tend to prevent penetration of light to the lower portions of the sensitive material, while U.S. Pat. No. 4,038,078 suggests that dyes can be used for control of photosensitivity, improvement of wave length selectivity, prevention of halation and the like. The combined teachings in this art neither indicate how the many requirements of the photostencil industry may be met, nor do they appear to have been used in that industry.
Another variant of the photostencil process is found in U.S. Pat. Nos. 3,510,303 and 3,507,654, which propose that certain advantages can be gained by the usage of a process in which a screen is placed in direct contact with a wet emulsion of a conventional kind. This is related to the direct and indirect photostencil techniques, but the light sensitivity and aging properties of the photosensitive materials still present the same problems, vacuum and printing frames are still proposed for use, and inordinately long drying times are still required. It should also be noted that it has been proposed by others, as in U.S. Pat. No. 3,730,715, to provide photographic images having continuous tone characteristics by using photopolymerizable materials, and to incorporate fillers in such materials for a broad range of generally specified purposes, and for the specific purpose of providing particular colors. Taken individually or together, all of these suggestions of known prior art do not reveal how a basic improvement in stencil screen processing, and in the stencil screens themselves, can be obtained.
Despite the many different investigations and efforts which have been undertaken toward the preparation of relief images using traditional photosensitizable layers and photopolymers, it is evident that in the present state of the art, the indirect, direct and direct/indirect methods of preparing photostencils are still predominantly used. The desirable objectives of increasing both the resolution and the durability of photostencils, while using a simpler and more reliable technique for preparing the screen and exposing the photosensitive layer, have not yet been satisfied by new materials and processes.